PURCHASES

PURCHASES:

31

The Mechanical Advantage gained by using a purchase is found by
counting the number of parts of rope at the moving block. This,
however, is merely the theoretical advantage as friction and the weight
of the block and rope are neglected.

Fig. 86.

Fig. 87.

Fig. 88.

Fig 89 Fig. 90.

Single Whip.—A rope rove through a single block fixed in any
position. No power is gained (Fig, 86).
Double Whip.—A rope rove through two single blocks—upper
block a tail block, lower one a movable hook block. Power gained—
double—that is weight of one unit on the hauling part will balance a
weight of two units on the hook block (Fig. 87),
Gun Tackle.—Two single blocks. Power gained two or three
according to which is the movable block. If the upper block in the
figure is the moving one the purchase is said to be rove to advantage
and the power gained is 3; but if the lower block is the moving one the
purchase is rove to disadvantage and the power gained is 2 (Fig. 88).
Handy Billy or Jigger.—A small tackle for general use; a double
block with a tail and single block with hook (Fig 89).
Watch Tackle or Luff Tackle.—Double hook block and single
hook block. If rove to advantage the power gained is 4, but if rove
to disadvantage the power gained ia only 3 (Fig. 90).